I have the same problem, for a few months now.
When I open my calendar tab in Chrome, I can add a flair to some calendar event.
After some time (but is it a matter of time only, not sure...) with Chrome running and the tab staying opened, I can't add flairs anymore. I still see the small "loading" red banner flashing (but maybe one time instead of two usually), but the event stays without any flair.
Then, if I press the "reload" button to refresh the calendar in the same tab, I can add flairs again.
This happens for Google flairs set as well as personal flairs set.